An apparatus for reconstructing an image signal of a foreground subject wherein a probe having a plurality of transducers for scanning a foreground subject by radiation, such as an ultrasonic beam, is connected to a phase detector through a receiver. An electrical signal corresponding to a wave reflected from the foreground subject is phase-detected by a phase detector. At least one resultant hologram signal is subjected to the A/D conversion to be stored in a buffer memory. A processing unit causes the hologram signal to be read out of the buffer memory, carries out the convolution integration of the hologram signal and the corresponding kernel function data by the synthetic aperture technique, thereby producing a reconstructed image signal. The processing unit further carries out the nonlinear compression of the amplitude of the hologram signal before the convolution integration and performs the nonlinear expansion of the amplitude of an image signal after the convolution integration.